BEFSR41 - certain sites are inaccessible

I recently noticed I can no longer get to www.usps.com or www.bankone.com. Pings just time out. After a lot of troubleshooting, I determined the router to be at fault. When I removed it from the picture and connected a PC directly to the cable modem, there was no problem. All users behind the router are affected the same.

So, what can be causing this? Has anyone else encountered it? Is there any known solution?